Support shmget(IPC_PRIVATE, …) on Windows

[cmb69]

We map the POSIX semantics of IPC_PRIVATE by creating unnamed file
mapping objects on Windows. While that is not particularly useful for
ext/shmop, which is the only bundled extension which uses shmget(),
and even constitutes a potential BC break for shmop_open(), it may
be useful for external extensions.

[nikic]

Can we test this?

[cmb69]

Can we test this?

I added a test to verify that private SHM is private. Adding a test to verify that private SHM can be shared between threads would likely require to have a suitable SAPI available.

[nikic]

Looks reasonable.

Do you think it would make sense to apply this to 7.3 as well? I see this as a bug fix in Unix emulation.

[cmb69]

I also see this as bug fix, and applying to PHP 7.3 would be helpful for APCu (and maybe some other extensions), but I'm somewhat concerned about the BC break for existing code calling shmop(0, …) without assuming this is supposed to be private. On the other hand, ext/shmop might be rarely used.

Somewhat related: shouldn't we introduce a constant to make IPC_PRIVATE available to userland? Say, PHP_IPC_PRIVATE.

[nikic]

I also see this as bug fix, and applying to PHP 7.3 would be helpful for APCu (and maybe some other extensions), but I'm somewhat concerned about the BC break for existing code calling shmop(0, …) without assuming this is supposed to be private. On the other hand, ext/shmop might be rarely used.

As they would already see that behavior on Linux, I don't think this is a problem. Using shmop() as Windows-only functionality seems somewhat unusual to me.

Somewhat related: shouldn't we introduce a constant to make IPC_PRIVATE available to userland? Say, PHP_IPC_PRIVATE.

Agree.

[cmb69]

Well, I think you're right, so I've filed https://bugs.php.net/79566.

For PHP_IPC_PRIVATE: see PR #5527.

[cmb69]

Applied as f33cf52. Thanks!